1. Field of the Invention
The present invention relates to a sheet feeding device including a sheet cassette on which sheets of various sizes can be loaded, and an image forming apparatus such as a copying machine including a sheet feeding device, a printer, a multifunctional machine including a copying function and a printer function or a fax machine.
2. Description of the Related Art
Recently, image forming apparatuses such as a copying machine, a printer, a multifunctional machine that includes a copying function and a printer function and a fax machine are widely used. A sheet cassette of the image forming apparatus of this type normally includes an intermediate plate provided in a cassette main body and pressure springs (helical compression springs) that serve as biasing members arranged between the cassette main body and the intermediate plate. The sheet cassette is configured so that each of the pressure springs applies a biasing force to the intermediate plate, thereby pressure-contacting a tip end of each of sheets loaded on the intermediate plate with a sheet feeding roller and separately feeding the sheets starting at an uppermost sheet loaded on the sheet cassette one by one.
Generally, if a height of the sheets loaded on the intermediate plate is large, a compression amount of each pressure spring is large. Conversely, if the height of the loaded sheets is small, the compression amount of each pressure spring is small. In other words, as the height of the sheets loaded in the sheet cassette is larger, pressure acting on the sheet feeding roller (sheet feeding pressure) when the intermediate plate presses a sheet against the sheet feeding roller increases.
To stably feed sheets, this sheet feeding pressure should fall in a constant range irrespectively of the height of the loaded sheets. To meet this demand, according to a conventional technique, a spring constant of each pressure spring is set appropriately, thereby keeping the sheet feeding pressure in the constant range irrespectively of the compression amount of each pressure spring (that is, the height of the loaded sheets).
However, a change in a gross weight of sheets due to a difference in size among the sheets loaded on the intermediate plate cannot be often dealt with appropriately only by setting of the spring constant. For example, if sheets equal to or larger in size than an A4 sheet or an A5 sheet such as a letter-size sheet (referred to as “large-size sheet”, hereinafter) and sheets equal to or smaller in size than a large-size sheet such as an A6 sheet (referred to as “small-size sheet”, hereinafter) are loaded by the same number, the small-size sheets are lighter in weight than the large-size sheets. Due to this, if the spring constant of each pressure spring is set to be fit to the large-size sheets, then the sheet feeding pressure is too high for the small-size sheets and multiple-feeding tends to occur. Conversely, if the spring constant of each pressure spring is set to be fit to the small-size sheets, then the sheet feeding pressure is too low for the large-size sheets because of the heavier weight of the large-size sheets than that of the small-size sheets, and non-feeding tends to occur. Further, a larger-capacity sheet cassette in which a large quantity of sheets can be loaded tends to have a fluctuation in the sheet feeding pressure.
Considering the problems, there is proposed a sheet cassette that includes rear end guides that can be detachably attached to an intermediate plate, that have different restricted heights according to sheet sizes and that restrict the number of sheets loaded on the intermediate plate as disclosed in Japanese Patent Application Laid-Open (JP-A) No. 10-77123. One of the rear end guides fit to one of a plurality of positioning holes is attached to the intermediate plate while the remaining or unused rear end guides are attached to holding holes, respectively. In this sheet cassette, the rear end guides are detached or attached for every sheet size and the height of the loaded sheets can be restricted so that the sheet feeding pressure acting on the sheet feeding roller falls within a constant range. Therefore, this sheet cassette can prevent sheet feeding failures such as multiple-feeding and non-feeding of sheets.
However, the conventional sheet cassette disclosed in the Japanese Patent Application Laid-Open No. 10-77123 has the following problems. It is necessary to detach or attach the rear end guides and to replace one rear end guide with the rear end guide fit to the sheet size whenever the sheet size is changed. Due to this, the conventional sheet cassette has poor operability. Furthermore, if a user forgets storing the rear end guides detached and unused in a preset storage area, the rear end guides are possibly lost or damaged. Moreover, when the sheet size is changed, the rear end guides are attached to or detached from the corresponding positioning holes provided in the intermediate plate, respectively. Due to this, it is disadvantageously impossible to load sheets of a size that does not correspond to any of the positioning holes provided in the intermediate plate.
The present invention, therefore, provides a sheet feeding device and an image forming apparatus that can restrict rear ends of sheets according to various sheet sizes in a stepless fashion while preventing sheet feeding failures such as multiple-feeding and non-feeding of sheets, and that can improve usability.